1. Field of the Invention
This disclosure relates generally to power distribution systems and more particularly, to a method and apparatus for a circuit protection system providing multiple zone protective functions for zone protection throughout the system.
2. Description of the Related Art
In power distribution systems, power is distributed to various loads and is typically divided into branch circuits, which supply power to specified loads. The branch circuits can also be connected to other power distribution equipment.
Due to the concern of an abnormal power condition in the system, for example, a fault or an overload, it is known to provide circuit protective devices or power switching devices, e.g., circuit breakers, to protect the circuit. The circuit breakers seek to prevent or minimize damage and typically function automatically. The circuit breakers also seek to minimize the extent and duration of electrical service interruption in the event of a fault.
A power distribution system includes different distribution levels. Power flow of each of the levels is controlled through a circuit breaker. In case of a fault, or overload, the circuit breaker nearest to the fault needs to be tripped as quickly as possible to disrupt power flow and avoid damage to the power distribution system. To ensure that only the required part of the system is disconnected, an upstream circuit breaker must be restrained from tripping for a finite time delay (or delay via an inverse time-current relationship) to enable a downstream circuit breaker to clear the fault. This introduces a time delay in the upstream circuit breaker trip time in case of a fault downstream resulting in a undesirably high let-through energy. Let-through occurs if an upstream breaker should have tripped, but the upstream breaker is configured to wait the time period to allow the downstream breakers to trip, the flow that continues during the time period is let-through.
Design of the power distribution system must establish trip settings for different breakers individually, so that the breakers are coordinated to have a trip time curve that delays trip time in an upstream direction of the distribution levels of the power distribution system or cascaded settings. FIG. 1 shows a schematic illustration of an examplary prior art power distribution system 10. Prior art power distribution system 10 includes a plurality of circuit breakers 12. One branch 17 of prior art power distribution system 10 includes a plurality of levels 14. The plurality of distribution levels include a first level 16 having circuit breakers included in a distribution board, a second level 18 having circuit breakers included in a sub-distribution switchboard, a third level 20 having circuit breakers included in a power distribution switchboard, and a fourth level 22 having circuit breakers included in a main switchboard. The circuit breakers of first level 16 are connected downstream of second level 18, the circuit breakers second level 18 are connected downstream of third level 20, and the circuit breakers third level 20 are connected downstream of fourth level 22 relative to power flow. For example, first level 16 includes 20-ampere circuit breakers, second level 18 includes a 100-ampere circuit breaker, third level includes a 400-ampere circuit breaker downstream of a 1200-ampere circuit breaker, and fourth level includes a 4000-ampere circuit breaker.
Such a contemporary system as prior art power distribution system 10 fails to clear a fault such that a fault continues to flow through the system until it passes through a breaker that is sensitive enough to detect the fault. Thus, as shown in FIG. 2, a larger upstream overcurrent device or circuit breaker 30 must be less sensitive and slower than a smaller downstream device or circuit breaker 32. Each of upstream circuit breaker 30 and downstream circuit breaker 32 are connected to an electronic trip device 31, 33 to provide overcurrent protection functions that provides logic and information processing to make trip decisions. FIG. 2 shows a fault current present in upstream circuit breaker 30 and downstream circuit breaker 32 on a log plot over a period of time from committing to trip to clearing the fault current. As shown by curve B1, upstream circuit breaker 30 does not trip until after a time period to allow time for downstream circuit breaker 32 to trip, shown on axis A1, during flow of the fault current, as shown by arrow F1. Sensitivity and speed must be undesirably dictated by coordination requirements, such as, the time delay between upstream breaker 30 and downstream breaker 32, rather than protection or safety requirements in a cascaded power distribution system. This increases the risk of damage to the system, such as failure to timely clear a fault, increasing let-through. As settings get bigger and the breakers get slower a time delay to detect flow and determine if the flow is too much increases while a breaker waits for a breaker downstream to trip. Minimizing the let-through is extremely desirable because let-through of a fault is dangerous and causes failure/melting of expensive components of a power distribution system.
Prior art zone selective interlocking (ZSI) systems only provide (yes/no) communication between two overcurrent devices in series. A bottom (load side) device or downstream circuit breaker 232 communicates to an upper (line side) device or upstream circuit breaker 230 whether it is reacting to a fault or not. When upstream circuit breaker 230 does not receive a signal that downstream circuit breaker 232 detects a fault it knows to accelerate itself to faster operation. Thus, as shown in FIG. 4, communication between a master or upstream circuit breaker 230 and a secondary or downstream circuit breaker 232, as shown by arrow Z, improves a time period, shown on an axis Time, between upstream circuit breaker 230, as shown by curve A1, and downstream circuit breaker 232, as shown by curve B1, interrupting power flow over larger upstream overcurrent device or circuit breaker 30 and smaller downstream device or circuit breaker 32 of prior art distribution system 10, as shown in FIG. 2, in a fault condition. Each of upstream circuit breaker 230 and downstream circuit breaker 232 has an electronic trip device 231, 233 to provide overcurrent protection functions that provides logic and information processing to make trip decisions. ZSI systems undesirably only allow improvement of delays between upstream circuit breakers and downstream circuit breakers as long as a fault is high enough to be over a predetermined pick up threshold and fails to increase sensitivity of upstream circuit breakers, such as upstream circuit breaker 230, due to tolerance and load sustaining needs over prior art system 10.
If the breakers have increased sensitivity, then the breakers may trip for faults that are another breaker's responsibility. Thus, there is a problem with protection versus coordination. Each breaker of system 10 makes independent decisions. This also decreases the efficiency of the system, such as through untimely opening of circuit breakers and nuisance tripping, and can increase the extent and duration of electrical service interruption in the event of a fault.
Accordingly, there is a need for circuit protection systems incorporated into power distribution systems that decrease the risk of damage and increase efficiency of the power distribution system. There is a further need for circuit protection systems that improve delays between upstream circuit breakers and downstream circuit breakers and increases sensitivity of upstream circuit breakers over the prior art.